There are many applications in which it is desirable to control the phase of a signal over a wide range of frequencies with nearly linear phase change with input control voltage (or current). It is also desirable to have that phase tuning sensitivity be nearly constant over the entire operating frequency range. This is useful when it is necessary to phase modulate signals over a wide range of frequencies with the goal of obtaining the same phase modulation independent of frequency for the same input stimulus. Another very desirable attribute is to have a range of phase adjustment greater than 360 degrees to be able to reach all possible phase states across the specified frequency range. In addition it is also very desirable to achieve the above described performance with relatively low and flat insertion loss.
One prior art phase shifter that attempted to address these problems is disclosed in U.S. Pat. No. 4,961,062. In this case an analog phase shifter is provided by cascading T high-pass filter and π low-pass filter sections. The high-pass filter section provide more phase shift as a function of input tuning voltage at the low end of the operating frequency range and less at the high end. The low-pass filter does the opposite providing more phase shift at the high end of the operating frequency range and less at the low end. When combined the two filters together may provide a “U-shaped” phase shift response vs. frequency with the most phase shift and highest phase tuning sensitivity occurring at the extreme low and high ends of the frequency range and the least phase shift and phase tuning sensitivity occurring in the middle of the frequency range. In the example given a variable phase shifter covering the frequency range of 6 to 18 GHz has a total phase range of ˜95° at either end of the band and ˜55° in the center.
The analog phase shifter of the '062 patent includes a T-section high-pass filter network and a π-section low-pass filter network. The low-pass π-network includes a fixed inductance element and a pair of variable capacitance elements connected in shunt, and the T-network includes a pair of variable capacitance elements connected in series and an inductive element connected in shunt. By cascade connecting the high-pass and low-pass filter sections, a composite phase shift characteristic may be obtained which forms a “U-shaped” phase shift response vs. frequency.
The user of such an analog phase shifter preferably ensures that the phase-frequency response of T-section high-pass filter is more abrupt than the π-section low-pass filter, and the overall phase shift over the bandwidth is not constant due to the “U-shaped” nature of the phase shift response vs. frequency. Also, in order to increase the total phase shift range, higher filter orders may be required for both high-pass filter and low-pass filter, which leads to an even worse phase shift variation over frequency when large overall phase shift is demanded.
The current analog phase shifters that use a low-pass filter section cascaded with a high-pass filter section suffer from certain disadvantages. Due to the “U-shape” phase-frequency response, the phase shift variation over frequency is typically large between the center of the frequency band and the high/low end of the frequency band. Also, these phase shifters also typically provide limited phase shift range at the frequency of interest, usually near the center of the frequency band.
Another prior art phase shifter that attempted to address the above problems is disclosed in U.S. Pat. No. 7,276,993. In this instance multiple all-pass networks tuned to different center frequencies are cascaded to provide a relatively flat response over a specified range of frequency.
The analog phase shifter of the '993 patent includes two or more cascaded all-pass sections with each section centered at a distinct frequency. Typically, the use of a single all-pass filter by itself would not provide a flat response over a range of frequencies, but by cascading multiple all-pass sections tuned at different center frequencies, the cumulative phase shift response curves results in a composite frequency-phase response that may provide a relatively flat phase over a specified range of frequencies.
There are also drawbacks from cascading multiple all-pass sections centered at different frequencies. Typically, these filters have degraded overall input/output return loss due to the narrowband nature of each single all-pass section. Two or more all-pass sections at the same (or similar) center frequency may be required to provide a necessary overall phase shift range due to the limited phase shift range for the single all-pass section. This may degrade the insertion loss by a factor of two or more.
Another method previously employed is to use a reflection-type phase shifter, as disclosed for example in U.S. Pat. Nos. 4,638,269 and 5,119,050. In such analog phase shifters, a pair of varactor diodes is serially connected to each of the phase shifting ports of a 3 dB hybrid coupler. However, the bandwidth of such type of analog phase shifter is limited and the insertion loss is generally high due to the limited bandwidth and the extra loss introduced by the 3 dB coupler.